Britain and the US are trying a new diplomatic initiative to find a way out of the Syrian impasse as the country sinks into a routine of daily violence that is threatening dangerous regional repercussions. It is a long shot that depends almost totally on Russian support and smacks of desperation. But nothing else is working.

Details are sketchy, but the scheme said to have been agreed by Barack Obama and David Cameron focuses on just one part of the six-point plan being promoted by Kofi Annan on behalf of the UN and the Arab League – "a Syrian-led political process" that must surely end in president Bashar al-Assad standing down.

It has the feel of a kite being flown – a fairly big one – designed to make an impression in Damascus and Moscow.

On a day that saw the International Red Cross pleading for access to help civilians trapped in Homs after 10 days of shelling, anything is worth trying. Opposition supporters said rebels and troops were still locked in fierce battles on Wednesday night. "They are still firing mortars and missiles into the city and there is heavy fighting," said activist Abu Yazen. Amid deepening gloom about the crisis, the US-UK move is designed to tempt Assad into negotiation. The novelty is that he is being offered the carrot of "safe passage" to attend a UN-sponsored conference in Geneva to discuss a new government and his exit. That would at least require suspension of the EU's travel ban.

Another far more significant – and highly controversial – element is being described as "clemency". The word is not quite right because, unlike Muammar Gaddafi last year, the Syrian president has never been referred to the International Criminal Court – as many believe he should be after 15,000 deaths in the last 15 months. Perhaps "immunity from prosecution" sounds too bald – even for an international community that knows it cannot possibly repeat the one-off Nato-led Libyan intervention and has no other good options available.

The model here is clearly the transition in Yemen, a very different country, where the veteran president Ali Abdullah Saleh was finally cajoled into stepping down in return for immunity for him and his family. Key relatives, however, remain in charge of a country that is barely more stable than before and whose fundamental problems have been unaffected by the dramas of the Arab spring. Yemen's powerful neighbour Saudi Arabia played a major political and financial role in that transition. None of Syria's neighbours have that kind of clout or cash. Indeed, its closest regional ally, Iran, is to be excluded from the proposed conference at the insistence of Washington and London, still pressing hard but without results over Tehran's nuclear programme.

British officials see some slim possibility that Russia, keen on that Yemeni parallel, will now help pressure Assad – though it has conspicuously failed to do so so far. Moscow, still smarting over Libya, has balked at anything that smacks of "regime change" though it would be hard to attach that label to something that was agreed by Syrians under an international umbrella.

The other very obvious snags are that Assad looks unlikely to bite while even the most pragmatic of his opponents – their numbers dwindling as the crisis deepens – would find it hard to believe he will negotiate in good faith. In an increasingly sectarian atmosphere, the president's departure would not solve the problem of his closest advisers and cronies as well as the wider Alawite community which dominates the security forces and fears revenge by the Sunni majority.

Annan will probably back this idea because pronouncing his peace plan dead would be too dangerous. But the bitter truth is almost all parts of it have been ignored: neither regime nor opposition are respecting a notional ceasefire; there has been no withdrawal of heavy weapons from cities; no free humanitarian and media access or significant releases of prisoners. The UN monitors, able only to report on but not prevent massacres, have all but given up.

In recent weeks the conflict has got worse, with the now routine use of artillery barrages, helicopter gunships and fully-fledged offensives. The fighters of the Free Syrian Army have better weapons being supplied by Saudi Arabia and Qatar – not enough to defeat Assad's forces but sufficient to keep fighting. Unknown terrorists have carried out bombings that bear the hallmarks of al-Qaida. Even cautious UN officials have characterised it as a "civil war". The Assad regime says it is fighting "armed terrorist gangs" supported by meddling Arabs, westerners and Israel. The opposition rejects the civil war label too, complaining that it establishes a false equivalence.

Assad, a senior Arab official said last week, now seems increasingly out of touch with reality. The question is whether there is still time and scope for international diplomacy.